1. Technical Field
The present disclosure relates to a toner, a developer, a process cartridge, and an image forming apparatus.
2. Description of Related Art
A method of manufacturing toner called dissolution suspension method is known. The dissolution suspension method includes the steps of dispersing a binder resin solution in an aqueous medium in the presence of a dispersant (e.g., a surfactant, a water-soluble resin) and a dispersion stabilizer (e.g., fine inorganic particles, fine resin particles) and removing the solvent by applying heat or reducing pressure. Japanese Patent Application Publication Nos. 09-319144 and 2002-284881 describe that toner particles prepared by the dissolution suspension methods have a uniform size without any classification treatment. In electrophotographic image forming apparatus, toner is required to be releasable from a heating member so as not to cause offset problem in that part of a fused toner image is adhered to the surface of the heating member and retransferred onto an undesired portion of a recording medium. Japanese Patent No. 3640918 describes that the offset problem can be solved by including a modified polyester resin in toner. Binder resin generally occupies 70% or more of toner composition. Most binder resins are derived from petroleum resources now being exposed to depletion. Petroleum resources cause a problem of global warming because they discharge carbon dioxide into the air when consumed. On the other hand, binder resins derived from plant resources have been proposed and used for toners. Because plant resources have incorporated carbon dioxide from the air in the process of growing, carbon dioxide discharged from plant resources is merely circulated between the air and plant resources. Thus, plant resources have the potential to solve the problems of both depletion and global warming. Japanese Patent No. 2909873 describes a toner including a polylactic acid as a binder resin. Generally speaking, polylactic acids have high crystallinity because the number of polar groups per unit structure is relatively large. Therefore, polylactic acids are considered not to be suitable for use in toner. Even polylactic acids having low crystallinity are considered not to be suitable for use in toner because they are poorly resistant to humidity conditions.
A toner including such a polylactic acid having low crystallinity may be difficult to stabilize its charge under low-temperature and low-humidity conditions or high-temperature and high-humidity conditions.
It is known that fine resin particles composed of styrene-acrylic backbones are used as the dispersion stabilizers in the dissolution suspension method. In this case, low-temperature fixability of the resulting toner particles is poor because the toner particles are covered with the fine resin particles composed of styrene-acrylic backbones. Such fine resin particles serving as dispersion stabilizers have a large amount of hydrophilic groups. Therefore, the fine resin particles cannot improve humidity resistance of the toner particles which may include a polylactic acid. Moreover, the surfaces of the toner particles which may include a polylactic acid are not completely covered with the fine resin particles and a part of the surfaces are exposed, which causes toner filming on carrier particles or charging members. In a case in which the surfaces of the toner particles are completely covered with the fine resin particles, low-temperature fixability further deteriorates. Therefore, it is generally difficult to make toner have low-temperature fixability, humidity resistance, and charge stability at the same time. Japanese Patent Application Publication No. 2010-122667 describes a toner including a resin having a polyhydroxycarboxylic acid skeleton obtained from optically-active monomers having an optical purity X of 80% by mole or less. The optical purity X is represented by the following formula:X(% by mole)=|X(L-form)−X(D-form)|wherein X(L-form) and X(D-form) represent ratios (% by mole) of L-form and D-form optically-active monomers, respectively. It is described therein that the polyhydroxycarboxylic acid skeleton can be formed from a polylactic acid (PLA).